1. Field of the Invention
The present invention relates to a high-yield-ratio and high-strength steel sheet (such as a cold rolled steel sheet, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet) excellent in workability. The invention relates particularly to a high-strength steel sheet having a tensile strength of 980 MPa or more and having a yield ratio heightened without being lowered in workability. The steel sheet of the invention is used suitably for, for example, members for household electric appliance, or structural members for automobiles (for example, body skeleton members such as a side sill, a pillar, a member, and reinforcing members; or strength members such as a bumper, a door guard, sheet members and suspension members), for which a high yield strength together with a high workability is required.
2. Description of Related Art
In recent automobiles, positive use has been made of a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip galvanized steel sheet (these may be collectively called a galvanized steel sheet hereinafter) for, e.g., their car body skeleton members, their reinforcing members and others, for which rust prevention is required. These steel sheets are required not only to have an excellent spot-weldability and a good workability but also to have an energy absorbing performance when an automobile using the sheets collides, so as to be high in yield strength, that is, yield ratio.
From the viewpoint of an improvement in the spot-weldability, it is effective to reduce the C content by percentage therein. For example, JP 2007-231369A discloses the use of a steel sheet wherein the C content by percentage is remarkably reduced into a value less than 0.1%. Although the reduction of the C content by percentage gives excellent ductility and other workabilityies to the sheet, the sheet is decreased in yield strength. Thus, there remains a problem that the sheet cannot attain compatibility between high yield strength and workability.
JP 2002-322539A discloses a thin steel sheet consisting substantially of a matrix of a ferrite simplex structure containing less than 0.10% of C, and fine precipitations dispersed in the matric and having a particle diameter less than 10 nm, and having a tensile strength of 550 MPa or more, thereby being excellent in press-formability. However, according to working examples described in this patent application publication, the tensile strength of the thin steel sheet is at most from about 810 to 856 MPa. Thus, the publication never discloses a steel sheet having both of a high yield strength and an excellent workability even when the steel sheet has a high strength of 980 MPa or more.
In the meantime, a typical example of a steel sheet having high strength and workability together is a dual phase steel sheet (DP steel sheet) made mainly of ferrite having a high elongation and martensite exhibiting a high strength. However, the DP steel sheet can gain only a low yield ratio so that the sheet cannot attain compatibility between high yield ratio and high workability. As the DP steel sheet, for example, JP 55-122820A and JP 2001-220641A each disclose a high-strength hot-dip galvanized steel sheet excellent in strength-ductility balance and others. However, according to these precedent techniques, the generation of martensite is caused in a cooling step after hot dip galvanization or alloying treatment. Thus, at the time of martensitic transformation thereof, moving dislocation is introduced into ferrite, so that the steel sheet is declined in yield strength.